1. Field of the Invention
The present invention relates to a switching regulator, and more particularly to a control method at the time of the start of a step-up switching regulator.
2. Description of the Related Art
Various electronic apparatus such as a portable phone, a PDA (Personal Digital Assistant), and a notebook-type personal computer of recent years have numerous devices that operate at different power supply voltages, such as light-emitting diodes (hereafter referred to as LED) disposed as a back light unit for a liquid crystal, microprocessors, and other analog and digital circuits.
On the other hand, an electronic apparatus like this has a battery such as a lithium ion battery mounted thereon as a power source. In order to supply the voltage output from a lithium ion battery to a device that operates at a different power supply voltage, one makes use of a DC/DC converter such as a switching regulator that boosts or lowers the power supply voltage.
For the step-up or step-down switching regulators, there are a method that uses a diode for rectification (hereafter referred to as a diode rectification method) and a method that uses a synchronized rectification transistor instead of a diode (hereafter referred to as a synchronized rectification method). The former case provides an advantage in that a high efficiency is obtained when a load current that flows through a load is small. However, this necessitates use of a diode in addition to an inductor and a capacitor on the outside of the control circuit, thereby increasing the circuit area. In the latter case, the efficiency obtained when the current supplied to the load is small will be inferior to that of the former. However, since one makes use of a transistor instead of a diode, it can be integrated in the inside of the LSI, thereby achieving scale reduction as a circuit area including the peripheral components.
Here, in the step-up switching regulator of the diode rectification method or the synchronized rectification method, a transistor for synchronized rectification and an inductor are connected in series between the input terminal receiving an input of the battery voltage and the output terminal outputting the boosted voltage (hereafter referred to as an output voltage). When a P-channel MOSFET is used for the synchronized rectification transistor and the back gate thereof is connected to the source (or the drain), there is a problem in that an electric current flows to the load via a body diode (parasitic diode) between the back gate and the drain (or the source) and the inductor even in a state in which the voltage-boosting operation is stopped by turning off the synchronized rectification transistor.
[Patent Document 1] Japanese Patent Application (Laid Open) No. 2004-32875
[Patent Document 2] Japanese Patent Application (Laid Open) No. 2002-252971
In order to prevent the current flowing to the load via the synchronized rectification transistor and the inductor during the stoppage of the voltage-boosting operation, one can consider a method of disposing a DC (direct current)-block transistor as a switching element on this current path. However, when this DC-block transistor is rapidly turned on, there will be a problem in that an inrush current flows.